While the radio base station is usually equipped with a frequency reference and an highly accurate clock, e.g. in the range of 0.05 ppm, the local oscillator of the mobile station has a lower accuracy (0.1 ppm), mainly for cost reasons. Also, the presence of two oscillators having the same accuracy degree will ending result not practical considering the sum of errors and the error due to the Doppler effect as well.
It is therefore essential that at the start of the connection the mobile station obtains an accurate synchronization from the base station.
In SCPC (Single Carrier Per Channel) systems the problem is solved with the use of a PLL (Phase Locked Loop) or similar system (for instance a so-called Costas Loop); it is sufficient that the mobile system has a frequency not far from the base one more than the link band to be able to link and correctly demodulate the signal received.
In TDMA systems this scheme cannot be proposed since the signal is not a continuous stream, but consists of time slots separated by guard periods during which the transmitter is off and a PLL will loose the link between one slot and the other one.
U.S. Pat. No. 4,653,075 describes a bit synchronizer for a BPSK input signal having a sinusoidal carrier frequency f that is much greater than the modulation signal bit rate. The synchronizer comprises means for repetitively sampling the amplitude of the input signal over a preselected sampling interval portion of each modulating signal bit period.
In a digital radiomobile system this synchronizer cannot be proposed since the signal is very disturbed and the amplitude sampling does not obtain acceptable results.